Among typical examples of known photochromic materials is silver halogenide. Silver halogenide heretofore used, e.g., for producing photochromic lenses in sunglasses discolors due to thermal vibration at ambient temperature on discontinuation of light irradiation, thus lacking a long-term stability in principle.
Also attempts have been made to use viologens as photochromic materials. In this case, the color development of univalent cation radical produced by reduction is utilized. However, with the progress of dimerization by the high reactivity of the color former itself, discoloration proceeds within hours while consuming viologen. Accordingly viologen also finds no use, in principle, as a material able to withstand a longterm use or repeated use.